Datasheet AD623 (Analog Devices) - 20

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AD623 Data Sheet
The circuit in Figure 45 must be built using a printed circuit
GROUNDING
board (PCB) with a ground plane on both sides. All component Because the AD623 output voltage is developed with respect leads must be as short as possible. The R1 and R2 resistors can to the potential on the reference terminal, many grounding be common 1% metal film units; however, the C1 and C2 problems can be solved by simply tying the REF pin to the capacitors must be ±5% tolerance devices to avoid degrading appropriate local ground. The REF pin must, however, be tied the common-mode rejection of the circuit. Either the to a low impedance point for optimal CMR. traditional 5% silver mica units or Panasonic ±2% PPS film capacitors are recommended. The use of ground planes is recommended to minimize the impedance of ground returns (and hence the size of dc errors). In many applications, shielded cables are used to minimize noise; To isolate low level analog signals from a noisy digital environment, for best CMR over frequency, the shield must be properly driven. many data acquisition components have separate analog and digital Figure 46 shows an active guard driver that is configured to ground returns (see Figure 47). All ground pins from mixed signal improve ac common-mode rejection by bootstrapping the components, such as analog-to-digital converters (ADCs), must capacitances of input cable shields, thus minimizing the be returned through the high quality analog ground plane. capacitance mismatch between the inputs. Maximum isolation between analog and digital is achieved by
+VS
connecting the ground planes back at the supplies. The digital
–IN
return currents from the ADC that flow in the analog ground
2 7
plane, in general, have a negligible effect on noise performance.
RG 1 100Ω 2 AD8031 AD623
If there is only a single power supply available, it must be shared
6 OUTPUT RG 5
by both digital and analog circuitry. Figure 48 shows how to
2 8 REF 3 4
minimize interference between the digital and analog circuitry.
+IN
5 4 As in the previous case, use separate analog and digital ground -0
–V
78
S
planes (reasonably thick traces can be used as an alternative to a 07 0 Figure 46. Common-Mode Shield Driver digital ground plane). These ground planes must be connected at the ground pin of the power supply. Run separate traces from the power supply to the supply pins of the digital and analog circuits. Ideally, each device has its own power supply trace, but these can be shared by a number of devices, as long as a single trace is not used to route current to both digital and analog circuitry.
ANALOG POWER SUPPLY DIGITAL POWER SUPPLY +5V –5V GND GND +5V 0.1µF 0.1µF 0.1µF 0.1µF 7 2 1 6 14 4 V AGND DGND AGND V DD 12 DD AD623 6 4 VIN1 ADC MICROPROCESSOR 3 AD7892-2 5 3 VIN2
046 8- 077 0 Figure 47. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies
POWER SUPPLY +5V GND 0.1µF 0.1µF 0.1µF 7 2 1 6 14 4 V AGND DGND AGND V DD 12 DD AD623 6 4 VIN1 ADC MICROPROCESSOR 3 AD7892-2 5
7 4 -0 778 0 0 Figure 48. Optimal Ground Practice in a Single-Supply Environment Rev. F | Page 20 of 26 Document Outline Features Applications General Description Functional Block Diagram Revision History Specifications Single Supply Dual Supplies Specifications Common to Dual and Single Supplies Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Applications Information Basic Connection Gain Selection Reference Terminal Input and Output Offset Voltage Error Input Protection RF Interference Grounding Ground Returns for Input Bias Currents Output Buffering Single-Supply Data Acquisition System Amplifying Signals with Low Common-Mode Voltage Input Differential and Common-Mode Range vs. Supply and Gain Additional Information Evaluation Board General Description Outline Dimensions Ordering Guide